1. Field of the Invention
The present invention relates generally to medical systems and methods. More particularly, the present invention relates to the use of diffractive optics for generating successive patterns of light energy for ablating corneal or epithelial tissue.
Photorefractive keratectomy (PRK) and phototherapeutic keratectomy (PTK) employ optical beam delivery systems for directing laser energy to a patient's eye in order to selectively ablate corneal tissue to reform the shape of the cornea and improve vision. All present commercial systems employ excimer lasers, where the beams from the lasers are spatially and temporally integrated in order to form a beam having uniform characteristics. In particular, the beams are integrated in order to display a flat intensity profile over a circular target region, often referred to as a "top hat" profile.
Once such uniformly integrated beams are achieved, they may be used in different ways in order to effect corneal ablation. In a first type of system, the beam has a width which generally corresponds to the desired target area on the cornea. The beam intensity is manipulated using an iris or other exposure control mechanism, and the desired corneal reshaping can be achieved by properly controlling the exposure. While highly effective and relatively easy to control, the need to employ a laser beam having a width equal to the treatment area (typically on the order of 5.0 mm to 10.0 mm) requires the use of large excimer lasers. Not only are such large lasers expensive, they also occupy a relatively large area , requiring significant space to house them.
As an alternative to such large beam diameter systems, laser "scanning" systems are also employed for corneal ablation. Such scanning systems employ a much smaller beam width, minimizing energy required from the laser. The smaller lasers are both more economic and require less space to house them. The use of a small beam width, however, complicates certain aspects of the treatment protocols. As most of the small treatment beams have a circular diameter, it will be appreciated that it is difficult to control exposure of the cornea. In particular, the beams overlap in non-uniform patterns as they are scanned over the cornea, making it very difficult to achieve properly controlled exposure over the entire target region. While elaborate control and exposure algorithms have been developed to minimize detrimental variations in exposure, none are entirely adequate.
For these reasons, it would be desirable to provide improved methods and systems for the scanning of light beams over corneal tissue in order to selectively ablate the tissue to treat vision disorders. In particular, it would be desirable to utilize small beam geometries with low power requirements while achieving an even energy distribution free from small regions of overexposure and underexposure. Moreover, it would be desirable to simplify the control schemes and systems required to scan small width light beams for corneal treatment. It would further be desired to provide treatment protocols and algorithms which are particularly suitable for accommodating the circular geometry of an ablation zone on the cornea. It would be still further desirable if the methods and systems could be used for the ablation of the epithelial tissue over the cornea prior to corneal treatment. At least some of these objectives will be met by the invention described hereinafter.
2. Description of the Background Art
Scanning systems for performing photorefractive keratectomy (PRK) and phototherapeutic keratectomy (PTK) are described in a number of patents, including U.S. Pat. No. 5,391,165. A laser surgical system employing a diffractive optical element adapted to an individual patient is described in U.S. Pat. No. 5,571,107. A laser scanning system employing unique reflective optics is described in U.S. Pat. No. 5,546,214. A temporal and spatial beam integrated for a PRK/PTK laser system is described in U.S. Pat. No. 5,646,791. The full disclosures of each of the above-cited U.S. patents are incorporated herein by reference.
Use of a diffractive optical element for integrating an excimer laser beam for use in PRK/PTK procedures is described in a co-pending application entitled LASER SYSTEM AND METHOD WITH DIFFRACTIVE OPTIC, U.S. patent application Ser. No. 09/015,841, filed on Jan. 29, 1998, the full disclosure of which is incorporated herein by reference.